The overall objective of this Phase I program is to demonstrate the feasibility of delivering intensity modulated breast brachytherapy, permitting balloon-based breast brachytherapy to better conform to patient anatomy, reduce potential toxicity by reducing skin dose, and permit this technique to be applied to a greater number of patients who would now be excluded due to suboptimal skin spacing. This will be achieved through the design of a series of applicators and a treatment planning strategy to facilitate employing them. Breast cancer is the most common malignancy among women in the United States. Breast conservation treatment, consisting of lumpectomy and whole-breast radiation therapy, has become the standard treatment option for patients with early stage cancer. However, Accelerated Partial Breast Irradiation (APBI), now being tested under an NSABP/RTOG Phase III clinical trial, can deliver an accelerated course of radiation therapy in 4-5 days of twice-daily treatments, thus significantly shortening the overall treatment duration and potentially reducing health care costs. This shortened treatment schedule decreases the burden of care for patients undergoing breast conservation treatment, eliminating the logistical problems for many patients, and making this option available to more women. Interest in the use of APBI accelerated with the development of the MammoSite balloon applicator. However, the dosimetric characteristics of the MammoSite are not entirely ideal. The skin dose with the MammoSite can be quite high, causing reason for concern as fibrosis, telangiectasias, and atrophic dermatitis are common late consequences of high skin doses. This technological innovation of applying carefully designed shielded applicators in conjunction with a low-energy HDR source to achieve intensity modulated breast brachytherapy will provide clinicians with a tool that can reduce skin dose, thereby reducing the probability of skin toxicities and permit more conformal dose distribution to breast brachytherapy patients. This development will permit many more patients to receive breast brachytherapy who would now be excluded because of concern for skin toxicity. Intensity Modulated Breast Brachytherapy System - PUBLIC HEALTH RELEVANCE: The clinical implementation of Intensity Modulated Breast Brachytherapy based on 169Ytterbium will benefit patients by reducing the probability of a variety of skin toxicities and permit many more patients to receive breast brachytherapy who would now be excluded because of concern for skin toxicity. Such a development would reduce the cost of radiation therapy for these patients, as the course of accelerated partial breast irradiation is much shorter. [unreadable] [unreadable] [unreadable]